1. Field of the Invention
The present invention relates to an optical cross-connect system used for optical wavelength-division multiplexed communication network. This application is based on Japanese Patent Application No. Hei 8-351247, the contents of which are incorporated herein by reference.
2. Description of Related Art
In a wavelength-division multiplexing transmission system, plural optical carriers having different wavelengths are respectively modulated using different signals, and these plural modulated optical carriers (i.e., plural optical signals) are multiplexed to be transmitted through one optical fiber. Therefore, at a junction point to which plural optical fibers are connected, an optical cross-connect system (or system) is necessary which can arbitrarily exchange plural wavelength-division multiplexed optical signals in each optical fiber, not depending on the wavelength of each signal.
FIG. 18 shows an example of the configuration of the conventional optical cross-connect system using the wavelength-division multiplexing techniques. Here, 4 pairs of input-output optical fibers are provided, in each of which 4 optical signals (.lambda..sub.1 -.lambda..sub.4) are wavelength-division multiplexed.
In FIG. 18, reference numerals 11-14 indicate input optical fibers, reference numerals 21-24 indicate 1.times.4 wavelength-division demultiplexers, reference numeral 30 indicates a 16.times.16 optical switch, reference numerals 711-744 indicate wavelength converters, reference numerals 81-84 indicate 4.times.1 wavelength-division multiplexers, and reference numerals 91-94 indicate output optical fibers.
The wavelength-division multiplexed light beams transmitted through input optical fibers 11-14 are respectively demultiplexed via wavelength-division demultiplexers 21-24 according to the wavelengths included in each wavelength-division multiplexed light beam. The 16 optical signals demultiplexed by 4 wavelength-division demultiplexers are introduced into any of 16 wavelength converters 711-744 via optical switch 30. Here, the optical signals introduced into wavelength converters 711-714 are respectively converted into optical signals having predetermined wavelengths of .lambda..sub.1 -.lambda..sub.4, and then multiplexed by wavelength-division multiplexer 81 to be output into output optical fiber 91. Similar operations are performed in other wavelength converters and wavelength-division multiplexers. In this way, it is possible to freely route optical signals (of each wavelength) transmitted through plural input optical fibers to plural output optical fibers, not depending on the original wavelength of each input signal.
However, in the arrangement shown in FIG. 18, if the number of the input-output optical fiber pairs and the number of the different wavelengths is large, it is difficult to construct optical switch 30. In particular, it is difficult to establish divided optical switches in order, for example, to cope with the increase of a pair of input and output fibers each time a demand occurs.
A cross-connect system in which the above problems are solved is disclosed in Japanese Patent Application, First Publication, Hei 3-219793 ("Wavelength division optical exchange"). A brief explanation of this conventional optical crossconnect system will follow. The operational principle of the optical switching part is that wavelength-division multiplexed light beams transmitted through m (m being an integer of 2 or more) input optical fibers are not demultiplexed and are directly distributed into plural m.times.1 optical switches, and each optical switch selects any one of the wavelength-division multiplexed light beams and any one optical signal (among the multiplexed optical signals) is further selected by a tunable wavelength selector.
FIG. 19 shows an example of the configuration of the conventional optical cross-connect system using such an optical signal selector. Here, 4 pairs of input-output optical fibers are provided, and in each optical fiber, 4 optical signals (.lambda..sub.1 -.lambda..sub.4) are wavelength-division multiplexed.
In FIG. 19, reference numerals 11-14 indicate input optical fibers, reference numerals 31-34 indicate 1.times.16 optical splitters, reference numerals 511-544 indicate 4.times.1 optical switches, reference numerals 611-644 indicate tunable wavelength selectors, reference numerals 711-744 indicate wavelength converters, reference numerals 81-84 indicate wavelength-division multiplexers, and reference numerals 91-94 indicate output optical fibers.
Each wavelength-division multiplexed light beam transmitted through input optical fiber 11-14 is split into 16 portions by each optical splitter 31-34 while maintaining the wavelength-division multiplexed state, and split light portions are introduced to 16 optical switches 511-544. For example, one of (16) outputs from each of optical splitters 31-34 is introduced into optical switch 511.
Any one beam output from the optical splitters is selected in each of 4.times.1 optical switches 511-544, and then in each of tunable wavelength selectors 611-644, a desired optical signal is selected from 4 optical signals which are wavelength-division multiplexed in the selected output. The optical signals selected via optical switches 511-514 and tunable wavelength selectors 611-614 are respectively introduced into corresponding wavelength converters 711-714 where each optical signal is converted into an optical signal having predetermined one of wavelengths .lambda..sub.1 -.lambda..sub.4. The converted (four) signals are multiplexed in wavelength-division multiplexer 81 and the multiplexed light beam is output into output optical fiber 91. In other wavelength converters and wavelength-division multiplexers, similar operations are performed. In this way, optical signals with each (predetermined) wavelength transmitted through plural input optical fibers can freely be routed to plural output optical fibers, not depending on the original wavelengths of the signals.
In the conventional cross-connect system shown in FIG. 19, optical splitters 31-34, optical switches 511-544, and tunable wavelength selectors 611-644 realize functions of wavelength-division demultiplexers 21-24 and optical switch 30 in FIG. 18. The construction of 4.times.1 optical switches 511-544 is simpler than that of 16.times.16 optical switch 30. In addition, the optical switches (511-544) and the tunable wavelength selectors (611-644) can be increased by one set for each pair of input and output optical fibers. That is, the optical switches can be increased step by step according to each demand.
However, in the publication, crosstalk with respect to 4.times.1 optical switches and tunable wavelength selectors of which the optical cross-connect system shown in FIG. 19 consists, or relevant optical switch driving circuits are not examined.
The 4.times.1 optical switches 511-544 in FIG. 19 can be constructed, as shown in FIG. 20, such that three 2.times.1 optical switching elements 151-153 are connected in 2-stage tree form. In this construction, when one of four inputs (1-4) is selected, switching operations regarding two 2.times.1 optical switching elements are necessary. That is, at least one optical switch driving circuit is necessary for each stage (see circuits 311 and 312 in FIG. 20). Generally, 2.sup.p .times.1 optical switch is constructed such that 2.sup.p -1 (total number) of 2.times.1 optical switching elements are connected in p-stage tree form and thus at least p optical switch driving circuits are necessary. Therefore, the size and the consumption power relating to the optical switch driving circuits are increased.
Additionally, optical switches 511-544 select a wavelength-division multiplexed light beam itself; thus, generated crosstalk includes a portion whose wavelength agrees with that of the (later-)selected optical signal. Such a state is shown in FIG. 21. In the figure, the bold arrow shows the passage of the selected wavelength-division multiplexed light beam. If there are crosstalk portions other than the selected multiplexed light beam, whose any wavelength agrees with that of the selected optical signal (refer to dotted arrows), beat noises are generated and the signal-to-noise (S/N) ratio is remarkably lowered. Therefore, high extinction ratios are required for optical switches 511-544.
According to Reference 1, Goldstein, et al., "Scaling limitations in transparent optical network due to low-level crosstalk", IEEE Photonics Technology Letters, vol. 7, pp. 93-94, 1995, when a beat noise is generated, crosstalk .epsilon..sub.b dB! for causing "power penalty" pp dB! (of sensitivity) at a bit-error rate (BER) is given by: EQU .epsilon..sub.b dB!=10 Log {(1-10.sup.-pp/5)/(4Q.sup.2)} (1)
where Q is a coefficient which is uniquely defined in accordance with the BER, for example, Q=7 at BER 10.sup.-12. Therefore, in order to suppress, for example, the power penalty at BER 10.sup.-12 less than 0.5 dB, it is necessary to suppress the crosstalk to be -30 dB or less. When the function of a 4.times.1 optical switch is performed using 2.times.1 optical switching elements connected as 2-stage form, a crosstalk component is added at each stage; thus, it is necessary to keep crosstalk of an optical switch below -33 dB.
On the other hand, for some kinds of optical switches, it may be difficult to realize the above-explained extinction ratio. For example, regarding a 2.times.1 optical switching element in the form of a silica-waveguide Mach-Zehnder interferometer using a thermo-optic effect, large crosstalk is generated at one of two input ports due to manufacturing defects with respect to the directional coupler. This problem will be briefly explained according to Reference 2, T. Kominato, et al., "Guided-Wave Optical WDM Circuits with Mach-Zehnder Interferometer Configuration", Technical Report of the IEICE, C-I, Vol. J73-C-I, No. 5, pp. 354-359, 1990.
FIG. 22 shows a basic configuration of the 2.times.1 optical switching element in the form of silica-waveguide Mach-Zehnder interferometer. This optical switching element comprises two directional couplers 161 and 162 and two single-mode waveguides 163 and 164 whose lengths are L and L+.DELTA.L, respectively. On one of the waveguides, thin film heater 165 is mounted, by which the temperature of a neighboring area of one waveguide is changed so as to change the effective refractive index via the thermo-optic effect and to perform the switching.
Regarding the above optical switching element, transmission efficiency T.sub.1 from port 1 to port 3, and transmission efficiency T.sub.2 from port 2 to port 3, are respectively given by: EQU T.sub.1 ={(1-k.sub.1)(1-k.sub.2)}.sup.1/2 -(k.sub.1 k.sub.2).sup.1/2 !.sup.2 +4{k.sub.1 k.sub.2 (1-k.sub.1)(1-k.sub.2)}.sup.1/2 sin.sup.2 (.pi.n.DELTA.L/.lambda..sub.s) (2) EQU T.sub.2 ={k.sub.2 (1-k.sub.1)}.sup.1/2 -{k.sub.1 (1-k.sub.2)}.sup.1/2 !.sup.2 +4{k.sub.1 k.sub.2 (1-k.sub.1)(1-k.sub.2)}.sup.1/2 cos.sup.2 (.pi.n.DELTA.L/.lambda..sub.s) (3)
where k.sub.1 and k.sub.2 are coupling efficiencies of light intensities with respect to directional couplers 161 and 162, "n.DELTA.L" indicates an effective optical path difference, and .lambda..sub.s means the wavelength of optical carrier. Here, propagation losses of the waveguides are assumed to be small enough to be omitted.
The coupling efficiencies k.sub.1 and k.sub.2 of directional couplers 161 and 162 are dependent on the relative refractive index difference and the distance of the two waveguides, and thus due to manufacturing defects thereof, the efficiencies k.sub.1 and k.sub.2 may depart from their design value 0.5. However, these manufacturing defects affect the two directional couplers almost equally; thus, it is relatively easy to realize the condition "k.sub.1 =k.sub.2 =k". In this case, the above formulas (2) and (3) are respectively simplified as: EQU T.sub.1 =(1-2 k).sup.2 +4 k(1-k)sin.sup.2 (.pi.n.DELTA.L/.lambda..sub.s)(4) EQU T.sub.2 =4 k(1-k)cos.sup.2 (.pi.n.DELTA.L/.lambda..sub.s) (5)
Here, effective optical path difference n.DELTA.L of the waveguide is designed to satisfy, for example, formula (6) in a state in which the thin film heater is not activated. EQU n.DELTA.L=.lambda..sub.s /2 (6)
In this case, transmission efficiency T.sub.1 from port 1 to port 3, and transmission efficiency T.sub.2 from port 2 to port 3, are independent of coupling efficiency k and defined as: EQU T.sub.1 =1 (7) EQU T.sub.2 =0 (8)
That is, when the thin film heater is not activated, this optical switching element outputs light input into port 1, having any wavelength but near .lambda..sub.s. In this operation, crosstalk from port 2 does not exist, in principle.
In addition, if the thin film heater is activated to increase the temperature of the neighborhood of one waveguide (164) and the effective refractive index of the waveguide is changed so as to change the effective optical path difference n.DELTA.L as below: EQU n.DELTA.L=.lambda..sub.s ( 9),
transmission efficiency T.sub.1 from port 1 to port 3, and transmission efficiency T.sub.2 from port 2 to port 3, are respectively defined as: EQU T.sub.1 =(1-2 k).sup.2 ( 10) EQU T.sub.2 =4 k(1-k) (11)
Therefore, by activating the thin film heater, this optical switching element chooses and outputs an optical signal input into port 2. However, if coupling efficiency k in this situation does not accurately agree with 0.5, crosstalk (1-2 k).sup.2 is generated from port 1. The above Reference 2 reports that if the distance between the two waveguides has, for example, 20% error (with respect to a design value) due to manufacturing defects, such crosstalk is worsened to approximately -16 dB.
On the other hand, FIG. 23 shows a construction of the tunable wavelength selector sometimes used in a conventional optical cross-connect system, where a wavelength-division multiplexed light beam is demultiplexed by wavelength-division demultiplexer 601 and one of the demultiplexed optical signals is selected and output via 4.times.1 optical switch 602. In FIG. 23, the bold arrow indicates the passage of the selected optical signal. When 4.times.1 optical switch 602 is constructed using 2.times.1 optical switching elements in multi-stage tree form, the size and the consumption power relating to the optical switch driving circuits are also large in this case.
However, optical switch 602 selects one from demultiplexed portions; thus, the crosstalk does not include a portion having a wavelength which agrees with that of the selected optical signal. In this case, no beat noise is generated. Therefore, regarding the power penalty due to the crosstalk, only an influence as the intensity noise should be considered. In this case, crosstalk .epsilon..sub.i dB! for causing power penalty pp dB! (of sensitivity) at a bit-error rate (BER) is given by: EQU .epsilon..sub.i dB!=5 Log {(1-10.sup.-pp/5)/Q.sup.2 } (12)
Therefore, in order to suppress the power penalty, for example, at BER 10.sup.-12 less than 0.5 dB, it is necessary to suppress the crosstalk to be -12 dB or less. When the function of 4.times.1 optical switch is realized using 2.times.1 optical switching elements connected in 2-stage form, it is necessary to keep crosstalk of each optical switching element to -15 dB or less.
As explained above, in the conventional optical cross-connect system, optical switches with a high extinction ratio must be selectively used for switches 511-544; thus, the cost becomes high. Additionally, if the tunable wavelength selector is also constructed using an optical switch, as explained with reference to FIG. 23, the system includes two different kinds of optical switches so as to meet the requirements for different extinction ratio levels. Therefore, integration of optical switches is difficult, and thus reductions of the size and the cost are also difficult. Furthermore, the number of necessary optical switch driving circuits is large. Therefore, the size and the consumption loss become large and thus miniaturization of the system may be impossible.